Solid electrolyte oxygen sensors and galvanic cell oxygen sensors are currently the most widely used sensors for monitoring gaseous oxygen. Solid electrolyte oxygen sensors typically include an electrolyte body made of an oxygen-ion-conductive ceramic such as zirconium (ZrO2) doped with traces of metal oxides (e.g., Y2O3). Porous electrodes on opposite faces of the body permit the diffusion of oxygen ions through the electrolyte body. Thus, when one electrode is exposed to a reference gas (e.g., air) and the other electrode is exposed to a sample gas (e.g., engine exhaust), a difference in oxygen partial pressure at the electrodes causes diffusion of oxygen ions that results in a corresponding voltage difference between the electrodes. A limitation of these solid electrolyte oxygen sensors is that the ceramic electrolytes used in the sensors only conduct oxygen ions when heated to temperatures above 400 to 600° C. Accordingly, the solid electrolyte sensors generally require time to heat up before becoming responsive, and an auxiliary electrical heater may be needed in the sensor as described in U.S. Pat. No. 4,175,019 to Michael P. Murphy.
Galvanic cell oxygen sensors using liquid electrolytes are generally simple, cheap, and operable at room temperature. U.S. Pat. Nos. 4,132,616 and 4,324,632 to Tantram et al., U.S. Pat. No. 4,495,051 to Fujita et al., U.S. Pat. No. 4,775,456 to Shah et al., U.S. Pat. No. 4,988,428 to Matthiessen et al., and U.S. Pat. No. 5,284,566 to Cuomo et al. describe some configurations for known galvanic cell oxygen sensors. Galvanic cell oxygen sensors operate on the same principle as a battery and generally include a cathode and an anode in contact with a liquid electrolyte. The cathode is typically made of a metal such as platinum, gold, or silver that is an effective catalyst for the electrolytic reduction of oxygen, and the anode is generally made of lead. The reduction of oxygen at the cathode produces oxygen ions that flow through the electrolyte to the anode and react with the lead anode. The resulting current between the cathode and anode is linearly proportional to the oxygen concentration, so that oxygen concentration measurements are easily determined from measurements of the current between the anode and cathode.
Galvanic cell sensors have several significant disadvantages. In particular, their life expectancy is a function of usage and the resulting anode consumption. Leakage of liquid electrolyte can also reduce sensor life and damage surrounding components. Furthermore, galvanic cell sensors have a tendency to read low due to loss in sensitivity as the sensors age. For most process control applications, false low oxygen readings can produce dire consequences. As a result, galvanic cell sensors must be frequently recalibrated, sometimes as often as once per day, depending on the criticality of the application. Another major drawback of galvanic cell sensors is their susceptibility to oxygen shock that results when galvanic cell sensors are exposed to a high concentration of oxygen. The high oxygen concentration can cause local variations of electrolyte composition that may take hours to equalize.
Polymer electrolytes have found applications in many areas of electrochemical technology. Nafion, for example, is a solid polymer electrolyte that has been widely used in the development of sensors and fuel cells. Nafion has an excellent ionic conductivity, outstanding chemical and thermal stability, and good mechanical strength. Additionally, Nafion, modified with a variety of noble metals, can create composite materials with ionic and electronic conductivity characteristics that are desirable for gas sensing. Galvanic cell sensors based on the solid polymer electrolyte (SPE) have been used to detect carbon monoxide (CO) and toxic gas with good performance as described in U.S. Pat. No. 4,227,984 to Dempsey et al. and U.S. Pat. Nos. 5,573,648, 5,650,054, 6,179,986, and 6,200,443B1 to Shen et al.
U.S. Pat. No. 4,227,984 to Dempsey et al. describes a gas sensor using three electrodes on the surface of a solid polymer electrolyte mounting membrane to detect toxic gas such as carbon monoxide (CO) or nitrogen dioxide (NO2). U.S. Pat. Nos. 5,573,648, 5,650,054, 6,179,986, and 6,200,443 B1 describe an SPE gas sensor with a two-electrode or three-electrode system for measuring carbon monoxide and other toxic gases in the environment. These toxic gas sensors generally are low cost and accurate and have a long useful life but have not been suitable for oxygen detection.
U.S. Pat. No. 6,080,294 to Shen et al. discloses a galvanic oxygen sensor including a solid polymer electrolyte and liquid electrolyte electrically connected in series between a cathode and an anode. This sensor is similar to the traditional galvanic cell oxygen sensor but uses the solid polymer electrolyte to control the rate of the electrochemical reaction. This can improve the useful life of the sensor, but the sensor still has many of the same problems as the more conventional galvanic cell oxygen sensors. In particular, using the liquid electrolyte still presents leakage problems. Another drawback is that a permeable membrane that controls oxygen diffusion can introduce larger temperature effects. Further, the lead (Pb) anode is consumed over time, changing the available surface area of the lead anode as lead (Pb) is converted to lead oxide (PbO and PbO2). As a result, the electrochemical activity and current output eventually falls to zero, at which point the sensor must be rebuilt or replaced, causing a lead pollution problem.
Another problem in galvanic oxygen sensors is the use of an alkaline electrolyte, which can be affected by prolonged exposure to acidic gases such as CO2. Most of these sensors should not be used continuously in atmospheres containing more than 25% CO2. In some cases, prolonged exposure to acid gas damages the basic sensor electrolyte. In other situations, high concentrations of acid gas produce a current flux that alters the normal expected output of the sensor at a given concentration of oxygen.
In view of the current state of sensor technology, an easily manufactured oxygen sensor having a long operational lifespan, fast response, low cost, no leakage, and a compact size would be highly desirable.